Recent years, for a window glass sheet for automobiles, a curved glass sheet having various shapes and curvatures is required in complying with a change in design. As a technique to produce such a curved glass sheet, there is a press-forming method to be conducted in a heating furnace. This method is suitable as a method for producing a curved glass sheet having a complicated shape or a deep-set shape because a glass sheet can be bent under an elevated temperature.
Heretofore, for a press-forming method to bend a glass sheet in a heating furnace, there has been known a forming apparatus wherein a forming ring is used as a lower mold on which the glass sheet is placed, the glass sheet deforming due to its own weight before press-forming, whereby preforming is carried out, and then, the glass sheet is pressed and bent on a forming plane of forming mold.
In such forming apparatus, the glass sheet heated nearly to its softening point while it is conveyed on rollers, is placed on the forming ring by means of a transfer machine having a positioning mechanism. The forming ring holding thereon the glass sheet is moved to a pressing position at a downstream side, meanwhile the glass sheet deforms due to its own weight on the forming ring. This deformation serves as preforming before the pressing operation. At the pressing position, the glass sheet is pressed and bent in a predetermined curved shape between an upper forming mold and the forming ring located at a lower position.
In this glass sheet forming apparatus in which the preforming is carried out by only deforming the glass sheet due to its own weight, however, it is insufficient to conduct the deep-set bending to a peripheral portion of a glass sheet. On the other hand, it is known to use a forming ring with a tilting mechanism in carrying out a deep-set forming operation. The forming ring with a tilting mechanism has a generally frame-like shape in plan view, made of metal, which comprises a fixed frame at the center and movable frames at its both sides. At both sides of the fixed frame, the movable frames of substantially U-like shape in plan view are pivoted with hinges. At the time of pressing a glass sheet in association with the forming mold, the movable frames are tilted so that the peripheral portion of the glass sheet can be bent certainly.
However, in such kind of forming ring with a tilting mechanism, the hinges project upward and accordingly, the projecting portions are apt to contact locally the glass sheet whereby there produce a portion formed by the contact with the forming ring and a portion without contact therewith, in the edge portions. These portions make it difficult to control the temperature distribution in the glass sheet and therefore, reproducibility and formability can not be stable. Further, a strain is apt to produce in a portion of glass sheet by the contact with a hinge, whereby the quality decreases.
The applicant has proposed an invention disclosed in JP-A-2003-335533. According to the invention disclosed in JP-A-2003-335533, a ring provided with an inner mold is disposed inside a forming ring with a tilting mechanism. A heated glass sheet is first supported on the inner mold whereby a certain degree of bending is conducted to the glass sheet due to its own weight. Then, the glass sheet is supported by the inner mold and an outer mold arranged outside the inner mold whereby the glass sheet is bent into a final shape. Thus, such two-stage forming operation suppress the occurrence of a strain in the glass sheet due to the contact with a hinge and it is possible to control the temperature distribution in the glass sheet.
Further, a displacement of glass sheet is apt to take place when a glass sheet is placed on the forming ring with a tilting mechanism. According to the proposed invention, such problem can be eliminated because the glass sheet is placed once on the inner mold and then, it is transferred.